In general, high efficiency transmission amplifiers used for wireless transmission apparatuses of mobile communications stations and the like have highly nonlinear characteristics. Hence, when a modulated signal for high-speed wireless communication is transmitted, such nonlinear distortion in transmission amplifiers causes out-of-band radiation power to be generated, influencing neighboring transmission channels.
In a typical transmission amplifier, the output becomes saturated with an increase in the input power, making it difficult to output a signal which is directly proportional to the input signal (refer to 1401 and 1402 illustrated in FIG. 14). Such a nonlinear characteristic of a transmission amplifier causes an unnecessary spectrum to be radiated outside of a signal band, for an input to the amplifier. This out-of-band radiation power causes the characteristics of other systems using the frequencies outside of the band to be degraded. In addition, an unnecessary spectrum is also radiated within the signal band. This causes the degradation in the characteristic of the signal itself.
Furthermore, since many of the recent digital modulation methods employ linear modulation, an amplifier having a nonlinear characteristic as described above is operated in a linear region which has low power efficiency so as to perform linear amplification.
As a method of suppressing out-of-band radiation caused by an amplifier, a predistortion technique is known in which the inverse characteristic of the amplifier is added in advance to an input signal to be input to the amplifier to compensate for the nonlinear distortion of the amplifier. This technique cancels distortion by distorting a signal in advance using a compensation signal generated based on the characteristic of the amplifier and then passing the signal through the amplifier. This allows the output signal of a transmission amplifier to have linearity (1401 illustrated in FIG. 14).
Furthermore, by using an adaptive predistortion method in which adaptive distortion compensation is performed by feedback of the output of an amplifier, optimal compensation is realized for the characteristic of the amplifier which varies among the individual differences of devices and varies with frequency, operation temperature, or the like.
FIG. 11 is a configuration diagram of an existing distortion compensation apparatus that uses an adaptive predistortion technique.
A predistortion signal generation unit 1102 within a distortion compensation unit 1101 adds, to a transmission signal, a compensation signal having the inverse characteristic of a transmission amplifier.
The output of the predistortion signal generation unit 1102, after being converted into an analog signal by a D/A converter (not illustrated), is upconverted by an orthogonal modulator 1107 to a carrier frequency using a signal output from a local oscillator 1108.
The modulator output signal is amplified by a power amplifier 1109 which is a transmission amplifier, and the output thereof is transmitted from a transmission antenna (not illustrated), via a coupler 1110. The output of the power amplifier 1109 is fed back from the coupler 1110 to the input side.
In other words, the output of the coupler 1110 is downconverted by a downconverter 1111 using a signal output from a local oscillator 1112. The output of the downconverter 1111 is passed through a filter 1113 and converted back into a digital signal by an A/D converter (not illustrated).
The resultant obtained feedback signal is input to an adaptive control unit 1103 that includes a predistortion signal generation unit 1104, an error computation unit 1105, and a coefficient updating unit 1106.
In the adaptive control unit 1103, the predistortion signal generation unit 1104 having the same configuration as the predistortion signal generation unit 1102 adds a signal for compensating for the nonlinear distortion characteristic of the transmission amplifier to the feedback signal.
The error computation unit 1105 computes the difference between the output of the predistortion signal generation unit 1102 and the output of the predistortion signal generation unit 1104, and outputs the difference as an error.
The coefficient updating unit 1106 updates the series operation coefficients to be set in the predistortion signal generation units 1102 and 1104 based on a convergence algorithm such as a least mean square (LMS) operation or the like so as to minimize the error signal output from the error computation unit 1105.
In this manner, the series operation coefficients are gradually made to converge to appropriate values, and the predistortion signal generation unit 1102 performs distortion compensation on an input signal x using these series operation coefficients having converged to the appropriate values. This allows the nonlinear characteristic of an analog circuit to be suppressed with high precision in a steady state, while maintaining power efficiency. Even when the nonlinear distortion characteristic changes due to the influence of temperature or frequency, the series operation coefficients are updated by the coefficient updating unit 1106 using the feedback signal so as to compensate for that change, thereby allowing a change in the characteristic to be dynamically compensated for.
As a method of distortion compensation used in the predistortion signal generation units 1102 and 1104, a predistortion method using a power series has been hitherto proposed. This is a method in which a compensation operation is performed by a series operation on a transmission signal x(t), as illustrated in FIG. 12.
A plurality of power operation units 1201 compute the powers of respective orders, such as the first power (X1), second power (X2), third power (X3), . . . , and n-th power (Xn), for the transmission signal x(t).
A plurality of coefficient multiplication units 1202 multiply the computation results of respective orders by corresponding series operation coefficients.
The outputs of the coefficient multiplication units 1202 are added together by an addition unit 1203 so as to constitute a predistortion signal y(t), which is output by the predistortion signal generation unit 1102 or 1104.
The above-described power operation is executed, in digital signal processing, as arithmetic processing expressed by Equation (1) described below.
                              y          ⁡                      (            n            )                          =                              ∑                          k              =              0                        3                    ⁢                                    ∑                                                l                  i                                =                0                            1                        ⁢                                          ∑                                                      l                    2                                    =                                      l                    1                                                  1                            ⁢                                                ∑                                                            l                      3                                        =                                          l                      2                                                        1                                ⁢                                                      ∑                                                                  l                        4                                            =                                              l                        3                                                              1                                    ⁢                                                            ∑                                                                        l                          5                                                =                                                  l                          4                                                                    1                                        ⁢                                                                                            h                                                                                    2                              ⁢                              k                                                        +                            1                                                                          ⁡                                                  (                                                                                    l                              1                                                        ,                                                          l                              2                                                        ,                            ⋯                            ⁢                                                                                                                  ,                                                          l                                                                                                2                                  ⁢                                  k                                                                +                                1                                                                                                              )                                                                    ⁢                                                                        ∏                                                      i                            =                            1                                                                                k                            +                            1                                                                          ⁢                                                                              x                            ⁡                                                          (                                                              n                                -                                                                  l                                  i                                                                                            )                                                                                ⁢                                                                                    ∏                                                              i                                =                                                                  k                                  +                                  2                                                                                                                                                              2                                  ⁢                                  k                                                                +                                1                                                                                      ⁢                                                                                          x                                *                                                            ⁡                                                              (                                                                  n                                  -                                                                      l                                    i                                                                                                  )                                                                                                                                                                                                                                                                (        1        )            
In Equation (1), x(n) denotes a transmission signal, h denotes series operation coefficients, and y(n) denotes a predistortion signal. Note that the above arithmetic processing is actually performed as processing for a complex signal.
When the distortion compensation processing in the predistortion signal generation units 1102 and 1104 is performed by a power series operation, as described above, the coefficient updating unit 1106 illustrated in FIG. 11 performs coefficient updating processing as follows. That is, the coefficient updating unit 1106 updates respective series operation coefficients (correction factors) h used in the predistortion signal generation units 1102 and 1104 such that the power series operation optimally approximates the nonlinear distortion amplification characteristic of the power amplifier 1109.
In this case, for the coefficient updating equations, such as those illustrated in FIG. 13, that are configured by using the error signal e(t) output from the error computation unit 1105, the coefficient updating unit 1106 updates respective series operation coefficients hi (i=1, 2, . . . , n), by executing a least mean square (LMS) algorithm for minimizing the error signal e(t). Here, μ denotes a step size parameter, and x(t), x2(t), . . . , and xn(t) denote the respective outputs of the power operation units 1201 illustrated in FIG. 12 within the predistortion signal generation unit 1102 or 1104.
FIG. 14 is a diagram illustrating the relationship between the amplifier characteristic of the power amplifier 1109 and the distortion compensation in the predistortion signal generation units 1102 and 1104, in the adaptive predistortion method of the existing distortion compensation apparatus using the above-described series operation.
The coefficient updating processing in the coefficient updating unit 1106 illustrated in FIG. 11 is performed so that the output power of a transmission signal at the time of updating each coefficient becomes optimum.
Referring to FIG. 14, it is assumed that the ideal amplifier characteristic of the power amplifier 1109 is a linear characteristic as denoted by 1401, and the actual amplifier characteristic is a nonlinear characteristic as denoted by 1402. In other words, regarding the amplifier characteristic 1402, there generally is a tendency for the amplification factor to become lower than that in the case of the ideal amplifier characteristic 1401 in a high-input-power range.
Here, the case is considered where the coefficient updating operation is performed when the input power (Ave-Peak) of a transmission signal is high, as illustrated as a range 1406 in FIG. 14. The distortion compensation curve corresponding to the power series operation using the respective operation coefficients obtained by this coefficient updating operation illustrates, for example, a characteristic denoted by 1403. This characteristic is represented by a curve that optimally compensates for the amplifier characteristic 1402 in the high-input-power range 1406.
On the other hand, the case is considered where the coefficient updating operation is performed when the input power (Ave-Peak) of a transmission signal is low, as illustrated as a range 1407. The distortion compensation curve corresponding to the power series operation using the respective operation coefficients obtained by this coefficient updating operation illustrates, for example, a characteristic denoted by 1404-1. This characteristic is an inverse characteristic that optimally compensates for the amplifier characteristic 1402 in the low-input-power range 1407.
Here, the case is considered where in a state in which the system is operating in the range 1406 in which the input signal power is high, a transmission signal having an input power in a range lower than the average power (Ave) of the range 1406 is input. In such a range, since the compensation curve 1403 at the time of high power is not significantly different from the compensation curve 1404-1 at the time of low power, the distortion compensation characteristic added to the transmission signal by the predistortion signal generation unit 1102 is close to optimal.
Conversely, the case is considered where in a state in which the system is operating in the range 1407 in which the input signal power is low, a transmission signal having an input power higher than the range 1407 is unexpectedly input. In the range 1407, the coefficient updating unit 1106 (FIG. 11) performs the coefficient updating processing without assuming a power higher than the range 1407. Hence, the compensation characteristic in the input power range 1405 higher than the peak power of the range 1407, is very likely to largely deviate from the inverse characteristic of the amplifier characteristic 1402, as illustrated by a compensation curve 1404-2 in FIG. 14. Consequently, in such a state, the distortion compensation characteristic added to a transmission signal becomes far from the optimal characteristic, thereby causing a high distortion power to be generated in the output of the power amplifier 1109, which causes the out-of-band radiation power characteristics to be degraded.
Thus, when a low-power transmission signal suddenly changes to a high-power transmission signal due to a power change, the existing technique does not allow distortion to be optimally canceled by using the series operation coefficients which have been generated up to that time.